A sensitive nerve when irritated transmits some influence to the
nerve-cell, whence it proceeds; and this transmits its influence,
first to the corresponding nerve-cell on the opposite side of the body,
and then upwards and downwards along the cerebro-spinal column
to other nerve-cells, to a greater or less extent, according to
the strength of the excitement; so that, ultimately, the whole
nervous system maybe affected.[5] This involuntary transmission
of nerve-force may or may not be accompanied by consciousness.
Why the irritation of a nerve-cell should generate or liberate
nerve-force is not known; but that this is the case seems to be
the conclusion arrived at by all the greatest physiologists,
such as Muller, Virchow, Bernard, &c.[6] As Mr. Herbert Spencer remarks,
it may be received as an "unquestionable truth that, at any moment,
the existing quantity of liberated nerve-force, which in an inscrutable
way produces in us the state we call feeling, MUST expend itself
in some direction--MUST generate an equivalent manifestation
of force somewhere;" so that, when the cerebro-spinal system is
highly excited and nerve-force is liberated in excess, it may be
expended in intense sensations, active thought, violent movements,
or increased activity of the glands.[7] Mr. Spencer further maintains
that an "overflow of nerve-force, undirected by any motive,
will manifestly take the most habitual routes; and, if these do
not suffice, will next overflow into the less habitual ones."
Consequently the facial and respiratory muscles, which are
the most used, will be apt to be first brought into action;
then those of the upper extremities, next those of the lower,
and finally those of the whole body.[8]

An emotion may be very strong, but it will have little tendency
to induce movements of any kind, if it has not commonly led to
voluntary action for its relief or gratification; and when movements
are excited, their nature is, to a large extent, determined by those
which have often and voluntarily been performed for some definite
end under the same emotion. Great pain urges all animals, and has
urged them during endless generations, to make the most violent
and diversified efforts to escape from the cause of suffering.
Even when a limb or other separate part of the body is hurt,
we often see a tendency to shake it, as if to shake off the cause,
though this may obviously be impossible. Thus a habit of exerting
with the utmost force all the muscles will have been established,
whenever great suffering is experienced. As the muscles of the chest
and vocal organs are habitually used, these will be particularly liable
to be acted on, and loud, harsh screams or cries will be uttered.
But the advantage derived from outcries has here probably come
into play in an important manner; for the young of most animals,
when in distress or danger, call loudly to their parents for aid,
as do the members of the same community for mutual aid.

[6] Muller (`Elements of Physiology,' Eng. translat. vol. ii. p. 932) in
speaking of the nerves, says, "any sudden change of condition of whatever kind
sets the nervous principle into action." See Virchow and Bernard on the same
subject in passages in the two works referred to in my last foot-note.

[8] Sir H. Holland, in speaking (`Medical Notes and Reflexions,'
1839, p. 328) of that curious state of body called the _fidgets_,
remarks that it seems due to "an accumulation of some cause
of irritation which requires muscular action for its relief."

Another principle, namely, the internal consciousness
that the power or capacity of the nervous system is limited,
will have strengthened, though in a subordinate degree,
the tendency to violent action under extreme suffering.
A man cannot think deeply and exert his utmost muscular force.
As Hippocrates long ago observed, if two pains are felt
at the same time, the severer one dulls the other.
Martyrs, in the ecstasy of their religious fervour have often,
as it would appear, been insensible to the most horrid tortures.
Sailors who are going to be flogged sometimes take a piece of lead
into their mouths, in order to bite it with their utmost force,
and thus to bear the pain.